1. Field of Invention
The present invention relates to an information viewing/listening system in which various types of information, such as music and movies, can be obtained via a network and viewed/listened to.
2. Description of Related Art
Services that enable a user to obtain information, such as music, movies, and games (referred to as content data) from a service center or the like via the Internet (hereinafter referred to as content data services) have become widely used.
A general content data service lets the user download content data stored at an information provider side (referred to as a service center) via the Internet, record the content data in a player having a function of at least playing content data (including not only a music player and a video player, but also a personal computer), and play the recorded content data. In such a content data service, a fee charged for using content data is generally paid by a credit card or the like via a network.
Since data handled in this type of content data service is digital data, the data can be copied almost without degradation. As such, the data may be transmitted without authorization and recorded into a unit owned by a user who has not paid a usage fee (unauthorized copying).
To address this problem, for example, a method is described in Japanese Unexamined Patent Application Publication No. 2001-67795.
This related art causes a plurality of information receivers having different pieces of registration information for using content data to exchange the registration information, thereby enabling the information receivers to mutually determine whether they are authorized to use the content data. Of the plural information receivers, a first information receiver that has the right to use the content data transfers this usage right to a second information receiver determined by the first information receiver as authorized to use the content data, thereby enabling the second information receiver having received the usage right to use the content data. Accordingly, content data received by the first information receiver is transmitted to the second information receiver, and the transmitted content data is recorded (copied) in the second information receiver.
This related art requires an information receiver to which content data is copied to exchange beforehand information with a service center or the like and to register registration information for using the content data. Even when the above-described second information receiver has a function of playing the content data (including a function of decrypting the content data since this type of content data is generally encrypted), if the second information receiver has no registration information, the data in the first information receiver cannot be copied to the second information receiver.
It is impossible for a unit having received content data (first information receiver in the example of the related art described above) to transmit this content data that has been copied to another unit (second information receiver in the example of the related art described above), and then the second information receiver makes a contract with a service center or the like and obtains registration information. Even an authorized copy to be made afterwards is restricted.
Since this related art imposes no restrictions on a decryption method for playing content data (in this case, decryption includes both decoding encoded content data and decrypting encrypted content data), when content data recorded in a memory (RAM or HDD, for example) of a particular information receiver is read in one way or another, the read content data itself can be copied without authorization to another unit, and the copied content data can be played on the unit having the unauthorized copy.
To decrypt encrypted content data, there is a related art method using a content key for decryption, which is received via a network. Since this content key is included in encrypted data, the encrypted data is rarely copied without authorization. However when the encrypted data is read in one way or another, the content data is decrypted without authorization.
To prevent unauthorized decryption, the content key is periodically updated. Every time the content key is updated, the encrypted content data must be re-received.
To enhance security, the content key must be updated very frequently. When the content key is updated very frequently, the encrypted data must be re-received many times. In this case, when the content data is large, as in a movie, an information receiver at the user side must perform a great amount of arithmetic processing every time it receives the content data. Since the transmitted data is large, the network load is high.
Practically, therefore the content key cannot be updated very frequently. The same content key must be used for a certain period of time. When a period of time before the content key is updated is excessively long, the content key and the content data may be copied to another unit without authorization.
As described above, a known content data service system including the above-described related art carries out various measures to prevent unauthorized copy of content data and to charge a legitimate fee, when viewed from the service center side. However, when viewed from the user side, user-friendliness is unsatisfactory, and there is much room for improvement.